The invention relates to the field of mobile phone systems. A subscriber in a mobile phone system, i.e. a subscriber station, for instance a mobile phone or some other means of communication, may be registered in a radio network or system, whereby it is registered in the radio network via system and traffic channels maintained by the base stations of the radio network.
In addition to the system channels in mobile phone systems, so-called direct mode channels can also be used in connection with a radio system, i.e. direct mode operation is applied. Subscriber stations using direct mode operation do not communicate directly with the radio network or its base stations. Direct mode channels are frequencies at which mobile phones or other means of communication are able to communicate directly with each other without the system.
Direct mode channels are typically used in situations where for instance, a group of portable mobile phones are communicating with each other at such a long distance from the base station that system channels cannot be used.
Another important way of using direct mode channels is to increase the capacity when the traffic in the system increases fast in some part of the service area of the system, for instance in some point-like part of the radio network.
A direct mode channel is referred to with the terms direct or simplex channel, or a simplex connection. A direct mode channel is a channel which is typically not at all used by the system. It may be for instance, a channel of the breadth of the channels of the system, for instance 12.5 kHz or 25 kHz. Among the mobile phones operating on a direct mode channel, the transmitting station has tuned its transmitter to the channel and transmits speech or data information. The other mobile phones set to direct mode operation have tuned their receivers to the same channel, whereby they are able to directly hear the transmission.
Operation on a direct mode channel may take place on the analog or digital modulation principle. A mobile phone transmitting on the channel may also transmit signalling information, such as information on rights of use and priorities or on the group operating on the channel. On the direct mode channel, an encryption may be carried out or plain speech can be transmitted.
Subscriber stations using direct mode operation communicate with other subscriber stations on a direct mode channel without necessarily being in direct contact with the base stations of the radio network.
One form of a direct mode channel is a direct mode channel equipped with a repeater, which direct mode channel comprises a separate repeater station in addition to the subscriber stations, this repeater station forwarding traffic between the subscriber stations using the direct mode channel. In this case, the traffic on the direct mode channel takes place on the semiduplex principle. The subscriber stations using direct mode operation can thus also communicate with the radio network via repeater stations. A repeater station is a piece of equipment comprising two radio apparatuses connected to each other. The repeater station transmits the information messages transmitted on the direct mode channel to desired network elements, for instance to the base stations of the radio network, these base stations transmitting the information messages further to the switching centres of the radio network and further to the databases located in the radio network.
In solutions according to prior art, a problem is presented, for instance by the fact that when a subscriber station, for instance a mobile phone or some other means of communication, starts to operate on a direct mode channel, the radio network or system which the subscriber has been registered in does not receive information on how information messages could be transmitted to the subscriber. The subscriber station that has switched to direct mode operation, i.e. to operate on a direct mode channel, cannot thus be reached by the radio network.
Another problem with the use of the solutions according to prior art is that when a subscriber station starts to operate on a direct mode channel, the radio network or mobile communication system loses its ability to control the operation of the subscriber station, i.e. the radio network is not able to command a subscriber station which has switched to the direct mode channel, to operate the way the radio network wants.
Yet another problem with the presented solutions according to prior art, is that when a subscriber station has switched to communicate on a direct mode channel, and the radio network is not aware of the location of the subscriber station, the radio network may page the subscriber to no avail on the system channels of its own service area, even though the subscriber station has moved outside the system. The base stations, radio paths, telecommunication resources and switching centres of the radio network are thus needlessly loaded when performing unnecessary subscriber paging.
Furthermore, yet another problem with the solutions of prior art is that they do not offer possibilities for identifying mobile phones operating on a direct mode channel. It is thus possible that such subscriber stations that do not have the radio system's permission to operate on a certain direct mode channel, nevertheless operate on this direct mode channel. These unauthorized subscriber stations may cause a great security risk especially in PMR systems (PMR=Private Mobile Radio).